Optical amplifiers in the form of erbium-doped fiber amplifiers (EDFAs) are expected to replace the current optoelectronic regenerators in many future optical long haul terrestrial and undersea lightwave communications systems. Optical amplifiers may be used as power amplifiers to boost transmitter power; as preamplifiers to increase receiver sensitivity; and, as repeaters to periodically boost the signal to a level sufficient for it to traverse the entire lightwave system.
Optical amplifiers are designed by considering a number of parameters including gain, output power, compression (i.e. gain saturation), and noise performance. Noise performance is typically measured by the noise figure which is defined as the signal-to-noise ratio at the input of the optical amplifier divided by that at the output. When optical amplifiers are used as repeaters, they should operate with very low noise figure and high output power in order to maximize the distance between adjacent repeaters in the lightwave system. For example, it would be very desirable, in future lightwave communications systems, to increase the distance between adjacent repeaters from the current 40 km to 100 km, or more. High output power is also required where repeaters are used in systems employing multiple multiplexed channels. In addition, repeaters must have sufficient gain to compensate for the loss in the optical fiber span between repeaters.
One prior art optical amplifier arrangement uses multiple EDFA stages to improve the gain characteristics of the optical amplifier. In this arrangement, two separate stages of amplification are separated by passive optical components. These passive optical components are required elements in most repeater arrangements, and may include isolators, filters, pump multiplexers, and the like. Typically in the prior art, passive optical elements are positioned at either the input or output of the optical amplifier. In this particular prior art arrangement, however, the placement of the passive elements between the two stages allows the multistage amplifier to have high gain while avoiding an increase in noise that would occur if the passive element were placed at the input of a single stage amplifier, or a loss of output power that would result of the elements were placed at the output of a single stage amplifier. Although the prior art multistage optical amplifier operates satisfactorily in certain applications, it has some limitations for use as a repeater in future long haul lightwave communications systems.